An improvement of chemiluminescent system for determination of peroxynitrite anion (ONOO^－) has been made. In this system, the effect of some antioxidants for scavenging ONOO^－ was tested. The constitute of this system and the program of starting were followed as: The ozone (O₃) was bubbled through a glass-frit into 10 ml 0.01 mol/L solution of sodium azide in CB(pH 10.5) to generate ONOO^－. The 800 μl ozonized solution of azide was injected into a glass tude in situ which contains 100 μl sample and 100 μl luminol solutions to initiate chemiluminescence (CL). The pluses / 6 seconds (CP6S) were determined immediately and continually for 10～30 times. A certain CL intensity (CP6S) was chosen as evaluation index to compare the activity of antioxidants. This chemiluminescent system is sensible, simple and stable. The determination limit was 8.74 μmol/L ONOO^－. The linear rang was 8.74～74.04 μmol/L ONOO^－. The intra batch and inter batch variation coefficient (CV%) of the analysis were 3.35%(n=10) and 5.52%(n=10) respectively. It was tested that Vit.C, teapolyphenol, procyanidin and thiourea all have effects on scavenging ONOO^－.